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Timber Decking
Guidance on some of the materials used to form roof decks can be found in Clause 6 of BS6229:1982.
Timber decking of all types should be specified to suit the load and span capability of the joists and purlins and is generally defined into two categories pre-formed sheets and timber boarding. Modern roof construction typically utilises sheets, but it is not uncommon in re-roofing situations to identify traditional timber boarding.
Sheet Boarding - Orientated Strand Board (OSB)
This is a wood panel composed of timber strands, orientated in cross directional layers. The choice of thickness is dependent on the span, type of insulation and membrane. For details on design criteria and installation, please refer to the panel manufacturer. Roofing grade OSB should be manufactured to BS EN300 1997 grade OSB/3 (formerly F2 of BS5669 Part 3.) and be certificated by the British Board of Agrement or WIMLAS. The minimum recommended thickness is 18mm.
Plywood
Plywood should be minimum 18mm thickness and certificated to conform to BS5268 Part 2: Structural use of timber - Code of practice for permissible stress design, materials and workmanship and to BS EN 636 Part 3: Plywood specification - requirements for plywood for use in exterior conditions. The Single Ply Roofing Association has researched the certification of plywood for roofing use and concluded that not all supply is suitably identified as conforming to a Standard. In these circumstances, it has concluded that one option is to specify to the APA certification scheme. In this case, grades C-D and C-C would apply.
Plywood decking should be installed at a moisture content of 14 -18% and laid with a gap between boards of 1mm per metre of panel size. Boards should not be laid with a moisture content in excess of 18%. Panels should be fastened securely to joists with ring shank nails or screws at 300mm centres.
Timber Boarding
New timber boarding should be a minimum 25mm thick and ideally, tongued and grooved, because this maintains its resistance to deflection after natural shrinkage.
Timber decking should be installed with no gaps at butt joints and securely fastened to joists with ring shank nails or screws.
Wood Wool Cement Slab
This is a rigid timber and cement based medium weight panel, which may be reinforced with steel edge channel for longer spans. Wood wool cement slabs should conform to BS 1105: ‘Wood wool cement slabs up to 125mm thick ‘ and to European Standard prEN 1368: Factory made wood wool products’. The choice of thickness is dependent on the span, type of insulation and membrane. For details on design criteria and installation, please refer to the panel manufacturer. If the panel is to be designed as a contributor to roof system thermal performance, then the effect of any metal channel support should be considered. The minimum required thickness is 50mm.
Woodwool slabs should be installed in accordance with the manufacturer’s requirement.
© 2006 Sarnafil Ltd. Legal Disclaimer
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